1. Field of the Invention
The present invention relates to a sensor unit with a piezoelectric sensor element for three-dimensionally sensing the magnitude and the direction of a physical quantity applied from the outside.
2. Description of Related Art
In the automobile, mechanical, and related industries, there is an increasing need for a sensor capable of accurately detecting a physical quantity such as force, acceleration, magnetic field, or the like. In particular, there is a need for a small-sized sensor capable of detecting such a physical quantity for each of two- or three-dimensional components. One known technique to realize such a sensor is to dispose a plurality of piezoelectric sensor elements on a flexible plate provided with an operating member (as disclosed in Japanese Patent Application Laid-Open No. 5-26744).
In this sensor, the flexible plate is deformed in response to a physical quantity applied from the outside to the operating member, and the piezoelectric element generates a charge corresponding to the deformation of the flexible plate thereby three-dimensionally detecting the magnitude and the direction of the physical quantity using the single sensor unit. (hereinafter, this type of sensor is referred to as a "three-axis sensor").
As an example of a three-axis sensor, an acceleration sensor of the type using a weight as the operating member is described below with reference to FIG. 2. As shown in FIG. 2, when an acceleration of a is applied from the outside on the sensor, a weight 1 experiences an inertial force f in a direction opposite to the acceleration a. As a result, a flexible plate 3, which extends in a horizontal direction between the weight 1 and a supporting base 2, has a deformation corresponding to the inertial force f.
Depending on the direction and the amount of the deformation 4, a corresponding amount of charges are generated in respective piezoelectric elements 5 disposed on the flexible plate 3. By detecting these charges, it is possible to perform a three-dimensional detection of the acceleration applied from the outside.
This type of sensor is described in further detail below with reference to FIGS. 3(a) and 3(b). In the sensor shown in FIGS. 3(a) and 3(b), the center of the bottom plane of a cylindrical weight 10 to which a flexible plate 12 is attached is defined as the origin O, the plane extending in parallel to the flexible plate 12 passing through the origin O is defined as an X-Y plane, and X and Y axes are defined in this X-Y plane so that X and Y axes are perpendicular to each other. Furthermore, a Z axis is defined so that it extends in a direction perpendicular to the X-Y plane and passes through the origin O.
In this structure, each portion of the piezoelectric material located between one pair of upper and lower electrodes is referred to as a "piezoelectric sensor element". In the specific example of the sensor shown in FIGS. 3(a) and 3(b), four piezoelectric sensor elements each consisting of a particular portion of the piezoelectric material and a pair of electrodes are disposed in the X and Y directions on the flexible plate 12, and additional eight piezoelectric sensor elements for use in detection in the Z direction are disposed.
In this sensor, the respective components of the inertial force f applied on the weight 1 by the external acceleration a are determined by the amounts of charge generated in the respective piezoelectric sensor elements as described below. That is, the X-axis component f.sub.x of the inertial force is detected by the piezoelectric sensor elements E.sub.1 -E.sub.4 as shown in FIG. 4(a). Similarly, the Y-axis component f.sub.y of the inertial force is detected by the piezoelectric sensor elements E.sub.5 -E.sub.8 (not shown). On the other hand, the Z-axis component f.sub.z, shown in FIG. 4(b), of the inertial force is detected by the piezoelectric sensor elements E.sub.9 -E.sub.12 and also by the piezoelectric sensor elements E.sub.13 -E.sub.16.
The directions of the respective components are determined on the basis of the charge polarity pattern. For example, in the example shown in FIG. 4(a), a charge polarity pattern of "+-+-" appears on the upper surface of the piezoelectric material and a charge polarity pattern of "+--+" appears on the upper surface of the piezoelectric material in FIG. 4(b), wherein the pattern is seen from left to right in both cases.
From the resultant force of the combination of the detected components f.sub.x, f.sub.y, and f.sub.z, the direction and the magnitude of the inertial force f and thus those of the external acceleration a can be determined in a three dimensional fashion using the single small-sized sensor.
In such a sensor, the flexible plate is required to have high flexibility so as to obtain sufficient sensitivity. On the other hand, the weight and the supporting base are required to have high rigidity and low flexibility.
Though it is possible to satisfy the aforementioned antipodal properties by assembling independently made members such as a weight, a supporting base, and a flexible plate into a sensor, many parts and manufacturing processes are required, thereby deteriorating productivity.
There is considered unitary formation of a sensor by the use of one material as a means for improving productivity by reducing the numbers of parts and manufacturing processes. As embodiments of unitary formation by the use of a ceramic material, there may be employed a method for cutting a ceramic green body formed by a one-axis press or the like with filling a ceramic powder into a mold as shown in FIG. 34, a method for filling a ceramic powder into a mold having a shape of mutual compensation with a sensor and then subjecting the ceramic powder to a one-axis press as shown in FIG. 35, a method for injection-molding a ceramic slurry as shown in FIG. 36, a method for molding by a slip casting, or the like.
However, since a unitary molded body has a low strength of a flexible portion and a wide variance in density in any of the aforementioned methods, it is difficult to make a flexible plate thin and to precisely control a thickness of the flexible plate. That is, a flexible plate has low flexibility and flexibility is varied depending on a flexible plate or a portion of a flexible plate in a sensor produced by an aforementioned method.
Therefore, in addition to a low sensitivity of a sensor, the sensitivity sometimes varies among sensors even if the same acceleration is applied. Further, in the case that sensitivity varies depending on an axis, the direction and the magnitude of the magnitude of the acceleration obtained from a resultant force become inaccurate, thereby deteriorating a sensor precision.
Further, in the sensor of the above-described type, if piezoelectric materials 5 are disposed in particular limited areas on the flexible plate 3 as shown in FIG. 5, then the thickness of those portions where no piezoelectric materials 5 are disposed becomes relatively thin, and thus these portions come to have a tendency to be deformed more easily. In contrast, the portions where the piezoelectric materials 5 are disposed are not deformed so easily. This results in a reduction in the sensitivity of the sensor.
Although it is possible to increase the sensitivity of the sensor by increasing the size of the weight so as to make the flexible plate deformable to a greater extent, this technique is unsuitable because such increasing in the size of the weight will result in an undesirable increase in the total size of the sensor, and thus it will become impossible to realize a sensor with a desired small size.